The present invention relates in general to a hydraulic internal gear machine of the type having a housing defining an intake port and a discharge port, a fluid displacement unit arranged in the housing, an outer gear supported for rotation about a central axis, an inner gear ring in a sliding engagement with the outer gear and supported for performing a wobbling movement in the housing about the center axis, the inner and outer gears defining a plurality of variable chambers therebetween; a flow control means including a rotary discs arranged for rotation at the same speed as the outer gear, one side of the disc forming a lateral boundary wall for the chambers, the rotary disc having a plurality of first openings communicating with the interstices between the teeth of the outer gear and having control edges at the other side of the disc, a stationary control plate adjoining the other side of the disc and being provided with a plurality of second openings cooperating with the control edges of the first openings, a part of the second openings communicating with the intake port and the other part of the second openings communicating with the discharge port.
When a machine of this kind is connected to a source of pressure fluid to convert its energy, about a half of the consecutive variable chambers resulting between the teeth of the inner and outer gears is under pressure whereby the side of the control plate adjoining the outer gear is subject to kidney-shaped or spheroid 1-shaped pressure field. The control openings in the rotary disc and in the stationary control plate are symmetrically distributed between the center axis and the periphery of the disc and of the control plate.
Approximately the entire periphery of the rotary disc is uniformly exposed to pressure fields. Due to the fact that different pressure fields build up on the side of the rotary disc adjoining the outer gear, the resulting pressure forces act eccentrically on the side of the disc adjoining the outer gear and concentrically on the opposite side when related to the center axis of the rotary disc. As a consequence, the rotary disc is subject to a tilting movement which has to be taken up by the housing and by the displacement unit. The rotary disc therefore within the limits of its play adjusts itself aplicably between the two parts of the machine whereby both the mechanical friction and the accompanying wear is increased and the volumetric efficiency of the machine due to uneven geometric play is impaired.